Method and system for compensating for track squeeze

ABSTRACT

A storage device includes a storage medium having a plurality of tracks of data. Reading apparatus for reading a current track of data that has been subject to previous encroachment to an extent from at least one adjacent track of data includes a write head that writes interfering data to the adjacent track of data while intentionally encroaching at least to that extent onto the current track of data, and circuitry that recovers data on the current track of data using the interfering data. A method for reading a current track of data, that has been subject to previous encroachment to an extent from at least one adjacent track of data, includes writing interfering data to the adjacent track of data while intentionally encroaching at least to that extent onto the current track of data, and recovering data on the current track of data using the interfering data.

CROSS REFERENCE TO RELATED APPLICATION

This claims the benefit of commonly-assigned U.S. Provisional PatentApplication No. 61/325,208, filed Apr. 16, 2010, which is herebyincorporated by reference herein in its entirety.

BACKGROUND

The background description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of theinventors hereof, to the extent the work is described in this backgroundsection, as well as aspects of the description that may not otherwisequalify as prior art at the time of filing, are neither expressly norimpliedly admitted to be prior art against the present disclosure.

This disclosure relates to a method and system for reading data that hasbeen recorded in an arrangement of tracks on a storage medium and isread by a read head that moves relative to the surface of the storagemedium. More particularly, this disclosure relates to compensating for atrack squeeze condition resulting from one or more off-track conditionsduring the writing of an adjacent track or tracks.

In magnetic recording, as one example of a type of recording in whichreading and writing are performed by a head that moves relative to thesurface of the storage medium, data may be written in circular tracks ona magnetic disk. To write data on a given track, the write head may becentered on that particular track. However, sometimes the write head maydeviate from its ideal path and stray “off-track,” with part of the headover the current track (i.e., the track to be written) and part of thehead over an adjacent track. This results in a portion of the data thatmay have been written previously on the adjacent track beingoverwritten. Later, when what had been the adjacent track is the currenttrack to be read, what had been the current track is now the adjacenttrack to the track to be read. At that time, the signal that is readfrom the now-current track may include inter-track interference (“ITI”)from the now-adjacent track. Moreover, if the track on the other side ofthe now-current track also had been written after the now-current trackwas written and also experienced an off-track condition during thatwrite operation, the signal from the now-current track may also includeITI from the other adjacent track, making it even more difficult toread. The encroachment on the current track from the adjacent tracks oneither side of the current track gives rise to a condition that may bereferred to as “track squeeze.”

If an ITI-generating event on a particular current track resulting froman off-track condition during writing of an adjacent track is detectedbefore the adjacent track is written again, then it may be possible torecover the data on the particular current track by using ITIcancellation, such as the ITI cancellation technique disclosed incopending, commonly-assigned U.S. patent application Ser. No.12/882,802, filed Sep. 15, 2010 and hereby incorporated by referenceherein in its entirety. However, many times it is the case that aparticular track will have been “squeezed” by multiple ITI events on itsadjacent tracks before an attempt is made to read that particular track.In such a case, ITI cancellation techniques such as those described maynot be useful, because the current data on the adjacent tracks may notbe the data causing interference on the current track, but rather thedata causing the interference may have been overwritten in the interimand at most only unusable remnants of those data may remain.

SUMMARY

An embodiment of a method for reading a current track of data, that hasbeen subject to previous encroachment to a respective extent from atleast one adjacent track of data, includes writing interfering data tothe at least one adjacent track of data while intentionally encroachingat least to the respective extent onto the current track of data, andrecovering data on the current track of data using the interfering data.

According to some embodiments, data from the at least one adjacent trackof data is buffered prior to writing interfering data, and buffered dataare restored to the least one adjacent track of data followingrecovering data on the current track of data. According to someembodiments, the interfering data are known interfering data.

According to some embodiments, the at least one adjacent track of dataincludes two adjacent tracks of data. A first one of the two adjacenttracks of data is on a first side of the current track of data, and asecond one of the two adjacent tracks of data is one a second side ofthe current track of data. The previous encroachment includes arespective extent of encroachment from each respective one of the firstand second ones of the two adjacent tracks, and the writing ofinterfering data includes writing interfering data to each respectiveone of the first and second ones of the two adjacent tracks whileintentionally encroaching at least to the respective extent from thefirst and second ones of the two adjacent tracks onto the current trackof data.

An embodiment of a storage device includes a storage medium having aplurality of tracks of data thereon. Reading apparatus for reading acurrent track of data that has been subject to previous encroachment toa respective extent from at least one adjacent track of data includes awrite head that writes interfering data to the at least one adjacenttrack of data while intentionally encroaching at least to the respectiveextent onto the current track of data, and circuitry that recovers dataon the current track of data using the interfering data.

Some embodiments include memory for buffering data from the at least oneadjacent track of data prior to writing interfering data, wherein thecircuitry restores buffered data to the at least one adjacent track ofdata following recovering data on the current track of data. Accordingto some embodiments, the interfering data are known interfering data.

According to some embodiments, the at least one adjacent track of dataincludes two adjacent tracks of data. A first one of the two adjacenttracks of data is on a first side of the current track of data, and asecond one of the two adjacent tracks of data is one a second side ofthe current track of data. The previous encroachment includes arespective extent of encroachment from each respective one of the firstand second ones of the two adjacent tracks, and the write head writesinterfering data to each respective one of the first and second ones ofthe two adjacent tracks while intentionally encroaching at least to therespective extent from the first and second ones of the two adjacenttracks onto the current track of data.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features of the disclosure, its nature and various advantages,will be apparent upon consideration of the following detaileddescription, taken in conjunction with the accompanying drawings, inwhich like reference characters refer to like parts throughout, and inwhich:

FIG. 1 is a simplified schematic view of a storage device with which thedisclosure may be used;

FIG. 2 is a view similar to FIG. 1 showing the formation of a simpletrack squeeze condition;

FIG. 3 is a view similar to FIG. 2 showing the formation of a morecomplex track squeeze condition;

FIG. 4 shows the writing of interfering data to overcome the tracksqueeze condition of FIG. 3;

FIG. 5 shows one iteration of an iterative process in accordance with anembodiment of the disclosure;

FIG. 6 shows another iteration of an iterative process in accordancewith an embodiment of the disclosure; and

FIG. 7 is a flow diagram of an example of an iterative method accordingto the disclosure.

DETAILED DESCRIPTION

This disclosure relates to a method and system for recovering data froma data track that has been “squeezed” by off-track recording of one ormore adjacent tracks, particularly when those adjacent tracks havesubsequently been overwritten so that the data causing the interferenceon the squeezed track are no longer available for use in recovering thesqueezed track.

FIG. 1 shows a simplified schematic view of a portion of a storagedevice showing three adjacent data tracks 101, 102, 103 on a storagemedium 100, with a write head 104 and a read head 105. Guard bands 106separate the tracks to prevent a small error in head positioning fromcausing unrecoverable damage to data previously written on an adjacenttrack. The distance 107 between the centerlines 116 of the guard bands106 may be referred to as the track pitch. Control circuitry 108controls the positioning of write head 104 and read head 105 via servomechanisms 109, and also controls the reading and writing of data.

If read head 105 is not properly centered on track to be read, then partof the signal it picks up may come from the guard band or the adjacenttrack, leading to signal-to-noise ratio degradation. However, there is asignificant margin of error allowed in read-head positioning becauseread head 105 is relatively smaller than write head 104 as compared tothe track width. Moreover, a misaligned read head will not overwritedata on an adjacent track, and if necessary the read operation cansimply be repeated with read head 105 properly aligned to obtain thecorrect data. On the other hand, if write head 104 is not properlypositioned, it can overwrite data on an adjacent track, and the marginfor error is smaller because write head 104 is relatively larger.

FIG. 2 shows how a track squeeze condition may occur. Here, track k(102) is the track of interest and was previously written, so that dataof interest are present on track k (102). Some time after the data ofinterest were written onto track k (102), a first set of other data 201were written onto track k−1 (101), and during that write operation,write head 204 was off-track, in the direction of track k (102).Similarly, some time after the data of interest were written onto trackk (102), a second set of other data 202 were written onto track k+1(103), and during that write operation, write head 214 was off-track, inthe direction of track k (102). This resulted in a “squeeze” of track k(102) so that only area 203 is free of interference, while area 213 issubject to interference from the track k−1 (101) data, and area 223 issubject to interference from the track k+1 (103) data.

It should be noted that while the two write operations 201, 202 areshown together in FIG. 2, it is not meant to imply that the twooperations necessarily occur simultaneously, or that the storage devicein question actually has two separate write heads 204, 214 that canperform two separate simultaneous write operations. Although such adevice may exist, write heads 204, 214 more commonly would represent twodifferent temporal instances of write head 104.

If the situation shown in FIG. 2 represented the maximum extent ofoverwriting of the data on track k (102), track k (102) could berecovered by using an ITI cancellation technique such as that describedin above-incorporated application Ser. No. 12/882,802, because the datafrom adjacent tracks k−1 (101) and k+1 (103) that caused the ITI wouldstill be present on those adjacent tracks to be used in the cancellationtechnique.

However, frequently the situation is that shown in FIG. 3, where each ofadjacent tracks k-1 (101) and k+1 (103) has been rewritten multipletimes 301, 302, 303, or 311, 312, 313, after the data of interest havebeen written onto track k (102). Because the ITI on track k (102) is theresult of that writing 301, or 311, that encroached furthest into trackk (102), and those data may no longer be available (or only small,unusable remnants might be available), the aforementioned ITIcancellation technique may not be able to be used. Even if the writing301, or 311, that encroached furthest into track k (102) were the mostrecent writing, if too much time had passed after that writing 301, or311, before an attempt to read track k (102), the data from writing 301,or 311, might not still be available (and might not be properly readablefrom its intended track k−1, k+1 (102, 103) because of interference fromtrack k (102)).

Therefore, in accordance with an embodiment of the present disclosure,and as shown in FIG. 4, data on the squeezed track (track k (102)) arerecovered by first buffering the current data on the adjacent tracksinto memory 118 (FIG. 1) and intentionally writing new, known,interfering data 401, 411 to the adjacent tracks, and then performing anITI cancellation technique such as that described above and in theabove-incorporated application Ser. No. 12/882,802. Afterwards, thebuffered data from the adjacent tracks may be restored from memory 400to those tracks.

The new known interfering data 401, or 411, should intentionallyencroach into track k (102) at least as far as the writing 301, or 311,that encroached furthest into track k (102), and preferably slightlyfurther, as shown, so that the ITI on track k (102) results only fromthe new interfering data 401, or 411.

It may not be known in advance how far the new interfering data 401, or411, should intentionally encroach on track k (102) in order to assurethat they are the only detectable contribution to ITI. If that is thecase, an iterative process may be used, according to which newinterfering data 401, or 411, are written in a first pass in which theyintentionally encroach by a minimum increment. An attempt is then madeto read track k (102). If the data on track k (102) cannot be readproperly (e.g., based on checksums, CRC, or other error-correctioncharacteristics of the data), the interfering data 401, or 411, arewritten again to adjacent track k−1 or k+1 (101, 103), but intentionallyencroaching further, and then another attempt is made. This processcontinues, with the intentional encroachment on track k (102) increasinguntil track k (102) can be properly read. According to one variant ofthis embodiment, the encroachment increases by same amount on eachiteration, and that amount may be the aforementioned minimum increment.

In an alternative embodiment, it may be possible to know from write headservo data (e.g., a position error signal), how far the furthestencroachment of write head 204, 214 onto track k (102) was. In such anembodiment, that servo data may be used to set the amount ofencroachment for writing interfering data 401, or 411, eithereliminating the need for iteration as described above, or at leastreducing the number of iterations by providing a more accurate startingpoint.

FIG. 5 shows that the encroachment positions of new known interferingdata 401, 411 may be only slightly offset from the positions of tracksk−1 (101) and k+1 (103) in a first pass of an iterative process. FIG. 6shows a later pass of an iterative process, with new known interferingdata 401, 411 reaching intermediate encroachment positions, with agreater degree of encroachment than in FIG. 5 but still not as far asencroaching data 301, 311. The encroachment positions of FIG. 6 may alsobe the initial encroachment positions where servo data are used toestimate the initial encroachment positions as discussed above. Notethat while encroachment or squeezing from both adjacent tracks is shownin the drawings, it may be possible to have encroachment or squeezingfrom only one of the adjacent tracks.

FIG. 7 is a flow diagram of an example of an iterative method 700according to this disclosure. The indicated method operations may beundertaken on either one or both of the adjacent tracks as may benecessary. At 701, method 700 starts with the write head moved to itsinitial increment off the adjacent track. As discussed, this may be adefault increment, or it may be determined by servo data. Next, at 702,an initial amount of interfering data are written to the adjacent track,and at 703 an attempt is made to recover data from the current trackbased on the interfering data written at 702.

At 704, the recovered data are examined for errors. If there are noerrors, the initial off-track increment was correct, the data are outputat 705 and method 700 ends. If at 704 there are errors in the recovereddata, then at 706 the write head is moved to a further increment off theadjacent track, additional interfering data are written at 707, and at708 an attempt is made to recover data from the current track based onthe interfering data written at 707.

At test 709, the recovered data are examined for errors. If there are noerrors, the new off-track increment was correct, the data are output at705 and method 700 ends. If at test 709 there are errors in therecovered data, then method 700 returns to 706 where the write head ismoved to another further increment off the adjacent track, and method700 continues until there are no errors at 709.

Thus it is seen that a data storage system, and method of decodingstored data, in which contributions from one or more adjacent tracks maybe accounted for in decoding one or more tracks of interest, has beenprovided.

It will be understood that the foregoing is only illustrative of theprinciples of the invention, and that the invention can be practiced byother than the described embodiments, which are presented for purposesof illustration and not of limitation, and the present invention islimited only by the claims which follow.

What is claimed is:
 1. A method for reading a current track of data thathas been subject to previous encroachment to a respective extent from atleast one adjacent track of data, the method comprising: buffering datafrom the at least one adjacent track of data; after the buffering,intentionally writing interfering data to the at least one adjacenttrack of data while intentionally encroaching at least to the respectiveextent onto the current track of data; recovering data on the currenttrack of data using the interfering data; and following the recoveringdata on the current track of data, restoring, to the least one adjacenttrack of data, the data buffered from the least one adjacent track ofdata.
 2. The method of claim 1 wherein the interfering data are knowninterfering data.
 3. The method of claim 2 wherein using the knowninterfering data comprises employing an interfering data cancellationtechnique that accepts known interfering data as an input.
 4. The methodof claim 1 further comprising determining extent of the previousencroachment from write head servo data.
 5. The method of claim 4wherein the writing interfering data and recovering data comprises:intentionally encroaching onto the current track of data by an initialincrement, writing an initial instance of interfering data to the atleast one adjacent track of data while intentionally encroaching ontothe current track of data by the initial increment, recovering initialdata on the current track of data using the initial instance ofinterfering data, checking for errors in the initial data, in absence oferrors in the initial data, outputting the initial data, and onoccurrence of errors in the initial data: intentionally encroaching ontothe current track of data by a further increment, writing a furtherinstance of interfering data to the at least one adjacent track of datawhile intentionally encroaching onto the current track of data by thefurther increment, recovering further data on the current track of datausing the further instance of interfering data, checking for errors inthe further data, and repeating the intentional encroaching onto thecurrent track of data by a further increment, the writing a furtherinstance, the recovering further data, and the checking for errors inthe further data, until there is at most a threshold level of error inthe further data, and then outputting the further data.
 6. The method ofclaim 5 further comprising determining the initial increment ofintentional encroachment from write head servo data.
 7. The method ofclaim 1 wherein: the at least one adjacent track of data comprises twoadjacent tracks of data; a first one of the two adjacent tracks of datais on a first side of the current track of data, and a second one of thetwo adjacent tracks of data is one a second side of the current track ofdata; the previous encroachment includes a respective extent ofencroachment from each respective one of the first and second ones ofthe two adjacent tracks; and the writing interfering data compriseswriting interfering data to each respective one of the first and secondones of the two adjacent tracks while intentionally encroaching at leastto the respective extent from the first and second ones of the twoadjacent tracks onto the current track of data.
 8. A storage devicecomprising: a storage medium having a plurality of tracks of datathereon; and reading apparatus for reading a current track of data thathas been subject to previous encroachment to a respective extent from atleast one adjacent track of data, the reading apparatus comprising: aread head that reads data from the at least one adjacent track of data;memory that buffers the data from the at least one adjacent track ofdata; a write head that, after the memory buffers the data from the atleast one adjacent track of data, intentionally writes interfering datato the at least one adjacent track of data while intentionallyencroaching at least to the respective extent onto the current track ofdata, and circuitry that recovers data on the current track of datausing the interfering data, and that following recovering data on thecurrent track of data, restores, to the least one adjacent track ofdata, the data buffered from the least one adjacent track of data. 9.The storage device of claim 8 wherein the interfering data are knowninterfering data.
 10. The storage device of claim 9 wherein thecircuitry that recovers data employs an interfering data cancellationtechnique that accepts known interfering data as an input.
 11. Thestorage device of claim 8 further comprising a write head servomechanism that records write head position data during the previousencroachment and uses the write head position data to determining therespective extent of intentional encroachment.
 12. The storage device ofclaim 11 wherein: the write head encroaches onto the current track ofdata by an initial increment, the write head writes an initial instanceof interfering data to the at least one adjacent track of data whileencroaching onto the current track of data by the initial increment, thecircuitry recovers initial data on the current track of data using theinitial instance of interfering data, the circuitry checks for errors inthe initial data, on occurrence of at most a threshold amount of errorin the initial data, the circuitry outputs the initial data, and onoccurrence of errors in the initial data: the write head intentionallyencroaches onto the current track of data by a further increment, thewrite head writes a further instance of interfering data to therespective one of the at least one adjacent track of data whileintentionally encroaching onto the current track of data by the furtherincrement, the circuitry recovers further data on the current track ofdata using the further instance of interfering data, the circuitrychecks for errors in the further data, and the intentional encroachingby the write head onto the current track of data by a further increment,the writing by the write head of a further instance, the recovering bythe circuitry of further data, and the checking by the circuitry forerrors in the further data, are repeated until there is at most athreshold amount of error in the further data, and then the circuitryoutputs the further data.
 13. The storage device of claim 11 wherein theinitial increment of intentional encroachment is determined from writehead servo data.
 14. The storage device of claim 8 the at least oneadjacent track of data comprises two adjacent tracks of data; a firstone of the two adjacent tracks of data is on a first side of the currenttrack of data, and a second one of the two adjacent tracks of data isone a second side of the current track of data; the previousencroachment includes a respective extent of encroachment from eachrespective one of the first and second ones of the two adjacent tracks;and the write head writes interfering data to each respective one of thefirst and second ones of the two adjacent tracks while intentionallyencroaching at least to the respective extent from the first and secondones of the two adjacent tracks onto the current track of data.